ISSUE-324: Prefer end to dur on generated out-of-line regions

Prefer end to dur on generated out-of-line regions

Prefer end to dur on generated out-of-line regions

The new process [generate out-of-line regions] in §9.3.2 always adds a dur attribute to the created region element R' even when dur attributes are not permitted. This situation is explicitly described in §10.2.3 for the case when a document instance specifies the use of the smpte time base and discontinuous marker mode.

I think (needs checking) that this could be resolved by specifying a computed end attribute in the root temporal extent in place of dur. If not, I guess we need to add more logic to carry across the appropriate combination of begin, dur and end for the content depending on the other factors in the document.